1. Field of the Invention
The present invention relates to a method for image reconstruction in a computed tomography (CT) device having a radiation source that can be displaced relative to an examination subject and a detector system for radiation emanating from the radiation source that registers projections, in different angular positions of the radiation source relative to the examination subject, of a region of the examination subject located within the measuring field, whose extent exceeds the measuring field, a projection being represented by a series of measuring points, each of which is characterized by a channel number and a measurement value.
2. Description of the Prior Art
In CT imaging systems the geometry of the measuring arrangement consisting of the radiation source, for instance an x-ray source, and the detector system defines a cylindrical measuring field centered on a rotational axis, around which the radiation source and possibly the detector system rotate in order to register projections. If parts of the examination subject that is to be projected are situated outside this measuring field, sharp image artifacts arise in the form of extensive bright regions and streaks at the margin of the measuring field near where the measuring field is exceeded. These artifacts are caused by measurement values at the beginning and/or the end of the projections that differ greatly from zero. The measuring field usually lies centered in a gantry opening. The circumstances of the measuring field being exceeded thus can be caused by either unusually large subjects or by mis-positioned subjects.
There are known dedicated methods for image reconstruction from segmented projections, such as iterative methods or wavelet methods, but these are characterized by high computing outlay.
It is also taught by U.S. Pat. No. 5,640,436 to counteract artifacts that are caused by the subject exceeding the measuring field by applying measuring data acquired by means of X-radiation of different energy levels.
In addition, U.S. Pat. No. 5,299,248 and European Application 0 030 143 teach methods to expand projections in which the measuring field is traversed by extrapolation such that each series of measurement points representing a detected projection is complete, beginning and ending with a measurement point whose measurementvalue is approximately zero. Despite this improvement, the image quality frequently leaves something to be desired.